Synthetic paper

ABSTRACT

A synthetic paper comprising a paper-like layer, which layer comprises a film-like matrix of a first resin and a reticulated structure consisting of fibrillated fibers made of a second resin dispersed in the matrix, and which layer contains a fine inorganic filler and voids therein. This paper is produced by drawing or stretching a drawable or stretchable sheet of a mixture of at least two resins and a fine inorganic filler at a temperature which is suitable for a first resin of the resin mixture and is less or not suitable for a second resin in the resin mixture thereby to produce a paper-like film comprising the matrix of the first resin and the reticulated structure of the second resin.

United States Patent 191 Toyoda Oct. 16, 1973 SYNTHETIC PAPER [7 51Inventor:

[73] Assignee: Kabushiki Kaisha Oji Yuka Goseishi Kenkyujo, Tokyo-To,Japan [22] Filed: Dec. 28, 1970 [21] Appl. No.: 101,574

[30] Foreign Application Priority Data Dec. 28, 1969 Japan 44/105111[56] 1 References Cited UNITED STATES PATENTS 3,097,991 7/1963 Miller etal. 161/169 X Tokashi Toyoda, Yokkaichi, Japan 1/1966 Blades et a1.260/25 12/1966 Magat et al 161/178 Primary Examiner-William A. Powell [57] ABSTRACT A synthetic paper comprising a paper-like layer, which layercomprises a film-like matrix of a first resin and a reticulatedstructure consisting of fibrillated fibers made of a second resindispersed in the matrix, and which layer contains a fine inorganicfiller and voids therein. This paper is produced by drawing orstretching a drawable or stretchable sheet of a mixture of at least tworesins and a fine inorganic filler at a temperature which is suitablefor a first resin of the resin mixture and is less or not suitable for asecond resin in the resin mixture thereby to produce a paper-1ike filmcomprising the matrix of the first resin and the reticulated structureof the second resin.

Patented Oct. 16, 1973 2 Sheets-Shut 1 FIG. 2

FIG. I

FIG. 4

FIG. 3

SYNTHETIC PAPER BACKGROUND OF THE INVENTION This invention relates tosynthetic papers consisting of a paper-like film or films and also to aprocess for producing such papers.

Recently, so-called synthetic papers consisting of paper-like films madeof synthetic resin have been proposed as substitutes for conventionalpapers made of cellulose fibers entangled together. In many cases, thepaper-like film is made of a synthetic resin treated to acquire paperycharacteristics. The synthetic papers may be made into a single layerstructure or laminated structure of paper-like layers each consisting ofa synthetic resin film treated to acquire the papery characteristics.

Although the paper-like layer can be obtained from a synthetic resinfilm treated to obtain the papery characteristics, the techniqueheretofore employed for acquiring these papery characteristics in thefilm has comprised dispersing a material opaque to light rays in thesynthetic resin film, and, as the material opaque to light rays, minutebodies of organic or inorganic substance (as an example of the formermaterial, a rubberlike polymer in granular form dispersed in apolystyrene matrix may be employed, and as an example of the lattermaterial, any suitable inorganic powder material is employed) and minutevoids or the like have been employed (it being considered that minutevoids created by drawing the sheet containing the minute bodies cancontribute much to the abovementioned opaque nature). Of course, theminute voids may also be created by employing a foaming agent admixed inthe sheet. In all cases, however the minute bodies or voids are ofsubstantially granular configuration, and these have served little forimproving the mechanical properties of the synthetic paper thusobtained.

SUMMARY OF THE INVENTION It is an object of the present invention toprovide a novel synthetic paper which has a reduced specific gravity,much improved whiteness, balanced tensile strengths in the longitudinaland traversing directions, better printability, and a better tactilefeel of the surfaces.

Another object of the present invention is to provide a novel processfor producing a synthetic paper including at leasta paper-like layer,wherein a reticulated structure consisting of fibrillated syntheticresin fibers is employed in at least one part of the material opaque tolight rays.

These and other objects of the invention are achieved by a novelsynthetic paper comprising at least one paper-like layer which is madeof a film-like matrix of a synthetic resin containing another type ofsynthetic resin and drawn or stretched in at least one direction so thata reticulated structure of fibrous shape is created in the other type ofsynthetic resin, and, furthermore, an inorganic fine filler of aquantity less than 60 percent by weight of the resin contents is addedto the matrix of the synthetic resin, whereby a void percentage of morethan percent is thereby obtained, rendering the paper-like layersubstantially opaque to light rays.

The term voids or void percentage is herein defined as 2 wherein thetrue specific gravity is the specific gravity of the material mixed withthe inorganic filler but not yet stretched to create voids.

According to the present invention in another aspect thereof," there isprovided a novel process for producing the novel synthetic paper, inwhich process, a mixture containing at least two extrudable or drawablesynthetic resins, the melting points or glass-transition temperatures ofwhich are different by at least 15C, and fine inorganic powder of aquantity less than percent by weight of these synthetic resins is formedinto a sheet-like configulation of drawable or stretchable nature, andthis sheet-like substance is thereafter drawn or stretched along atleast one direction thereof at a temperature which is suitable fordrawing or stretching the synthetic resin component having a lowermelting point or glass transition point but is lower than the optimumdrawing temperature of the other synthetic resin having a higher meltingpoint or glass transition point.

The terms melting point and glass-transition temperature are hereinapplied respectively to crystalline and non-crystalline resins of thedrawable or stretchable synthetic resins.

According to the present invention in'still another aspect thereof,there is provided another mode of producing synthetic papers, wherein amixture comprising essentially at least two drawable synthetic resins ofmelting points or glass transition temperatures which differ by at least15C and fine inorganic powder of a quantity less than 60 percent byweight of these synthetic resins is formed into a laminated structurestretchable or drawable sheet of the mixtures of the resins, and thislaminated sheet is thereafter drawn or stretched in at least onedirection at a temperature which is suitable for drawing or stretchingthe synthetic resin having a lower melting point or glass transitiontemperature but is lower than the optimum drawing temperature of theother synthetic resin having a higher melting point or glass transitiontemperature.

The nature, principle, and utility of the present invention will be moreclearly understood from the following detailed description of theinvention when read in conjunction with the accompanying photographs anddrawing.

BRIEF DESCRIPTION Omit ILLUSTRATIONS 60 MISETAILED nsscmiidiibi-"iiiINvI shrioh The principal feature of the present invention is that thedifference in the behaviors of two or more synthetic resins due to thedifference in optimum drawing temperatures thereof is advantageouslyutilized. More specifically, when a mixture of these synthetic resins ina state of a non-oriented sheet is drawn or stretched at a certaintemperature, the first synthetic resin whose optimum drawing orstretching temperature corresponds to this drawing temperature is drawncontinuously into a drawn film, whereas the second synthetic resinhaving an optimum drawing temperature higher than the abovementionedtemperature is fibrillated by the shearing force or tensile force causedat the time of drawing or stretching because of the coexistence of theeasily drawable first synthetic resin, and, furthermore, the resultingfibrillated fibers are reticulated and dispersed into the drawn film ofthe first synthetic resin.

Generally speaking, the drawing temperature utilized in this productionprocess according to the present invention is of a value suitable fordrawing the first synthetic resin but is lower than the optimum drawingor stretching temperature of the second synthetic resin. Although atemperature suitable for drawing the first synthetic resin is related tothe lower limit of the temperature range employable in the drawingprocess according to the invention, the temperature suitable fordrawing, the first synthetic resin is of a value nearly equal to or alittle higher than the optimum drawing temperature of the firstsynthetic resin (if the drawing temperature is too high, the drawn filmwill melt and break (reference: Example 5 set forth hereinafter, therebeing an allowable range of the optimum drawing temperature.Accordingly, the lower limit of this allowable temperature range definesthe lower limit of the drawing temperature employed in the process ofthis invention.

On the other hand, the upper limit of the drawing temperature used inthis invention coincides with the optimum drawing temperature of thesecond resin. Since the second synthetic resin having a higher meltingpoint is drawn under a restrained condition, the second synthetic resinis reticulated. For this reason, the drawing temperature employed in theinvention cannot be a temperature higher than a temperature at which thesecond synthetic resin is easily elongated, in other words, the optimumdrawing temperature, whereby the upper limit of the optimum drawingtemperature range for the second synthetic resin defines the upper limitof the drawing temperature range employed in this invention.

Since the synthetic paper consisting of one or more paper-like layers iscaused to be a reticulated structure comprising one drawn film-likemateix (preferably in two directions) ofa synthetic resin andfibrillated fibers (having a diameter of from 0.5 to microns) of asecond synthetic resin dispersed in the first synthetic resin, thesynthetic paper according to the present invention is quite different inits structure from the conventional synthetic paper which has beenproduced by bonding fibrillated fibers together in a form similar tounwoven cloth. The above described matrix of the paper-like layeraccording to the present invention is not necessarily of a solidstructure, and, preferably, the matrix contains minute voids therein.The voids are created by drawing the film containing minute inorganicpowder as described hereinafter, and the reticulated structure dispersedin the matrix in itself also contains extremely minute voids.

As a result, the synthetic paper according to the present invention, ina preferred form thereof, is provided with the following characteristicfeatures.

1. A lower specific gravity The synthetic paper has a lower specificgravity than that obtained merely by stretching a film containinginorganic minute powder. The reason for this is believed to be in theexistence of minute voids contained in the reticulated structureincluded in the resin matrix.

2. Improved whiteness and opacity This is believed to be caused also bythe minute voids existing in the reticulated structure.

3. Balanced longitudinal and lateral strengths.

4. Smoother and more lustrous surface of the synthetic paper.

5. Excellent printability of the paper.

6. Excellent strength and opacity even when made thinner.

7. Excellent workability for producing secondary products. A typicalsynthetic paper having the above described features can be obtained byinitially admixing two kinds of synthetic resins, the melting points orglass transition temperatures thereof (hereinafter called simply themelting points) being different from each other by more than 15C, addingthereto an inorganic powder so that a non-oriented sheet (in broadersense, this is defined as drawable sheet) is thereby formed, and drawingor stretching the sheet in two different directions or axes in the planeof the sheet.

More specifically, the two kinds of synthetic resins to be mixed intothe above described non-oriented sheet should have melting points, inthe above stated sense, differing from each other by more than 15C. (Ifthe difference of the melting points is less than 15C, no fibrillationwill be caused). The upper limit of the difference of the melting pointsis held to about 100C, and in ordinary cases, a combination of syntheticresins having melting points differing from 20C to 50C between eachother is employed for forming the nonoriented sheet. Furthermore, forthe purpose of obtaining the reticulated structure consisting of minutefibrillated fibers uniformly distributed in the first synthetic resin,the first and the second synthetic resins must be admixed homogeneouslybefore the resins thus mixed are formed into the non-oriented sheet andthen drawn. This means that two kinds of resins which are of compatiblenature (mutually soluble) or are capable of forming a fine, non-uniformdispersion are employed, and for the synthetic resin having a highermelting points, any of those which are crystalline or are easilycrystallized are preferred.

It has been found that highly crystalline polyolefins (such aspolyethylene, polypropylene, and the like), polyamides, polyesters(particularly polyethyreneterephthalate), are suitable for use as thesynthetic resin of higher melting point, and that low or noncrystallinepolyolefins (such as olefin-copolymers, polybutene-l and the like),vinyl resins (polyvinylchloride, polyvinylidene chloride, and the like),styrene resin (polystyrene or styrene-polymer with its ring or sidechain substituted), or copolymers of monomers constituting theabovementioned high-melting-point of low-melting-point polymers aresuitable.

Since the difference of the melting points is a relative quantity, anycombination of resins can be selected from the abovementioned groupshaving high melting points and low melting points as long as the desireddifference can be obtained. The mixing ratio of the two component resinsis preferably determined so that the content of the lower melting pointresin is from 15 to percent by weight of the resultant mixture. If thecontent of the lower melting point resin is less than the stateddescribed range, the drawing of the mixed sheet becomes impossible inmany cases.

The inorganic powder employed in the synthetic paper according to thepresent invention is required for the formation of voids in the matrixphase and the reticulation of the synthetic resins and is also effectivein promoting the fibrillation of the high melting point synthetic resinincluded in the synthetic paper. This inorganic powder may be any of thekinds used in the field of resin production as an inorganic filler, butit is preferable that the powder have a heat stability sufficient towithstand the temperature at which the mixed sheet is extruded and drawnor stretched. Furthermore, this inorganic powder is preferably ofsufficiently minute size, that is, of a grain size in a range of from0.1 to 5 microns, and the mixing proportion of the powder is preferablyless than 60 percent by weight. When the mixing ratio of the inorganicpowder is more than the abovementioned amount, the drawing, or stretchigof the mixed sheet is also made difficult. More practically, theinorganic powder may consist of calcium carbonate (of heavy andprecipitated type), aluminum silicate compound (such as clay ofagalmotolite or kaoline), silica compound (such as diatom), bariumsulfate compound, titanium oxide compound, talc, or the like.

A mixture of the above described synthetic resins and inorganic powderis thereafter formed into a film-like structure drawn in its twoorthogonal axes. While the biaxially drawn or stretched film may beproduced from the above described mixture directly in one step, it ismore ordinarily produced by once forming the mixture into a sheet ofsubstantially nonoriented nature which is thereafter biaxially drawn orstretched. The formation of the nonoriented sheet may be carried out byany arbitrary or conventional procedure such as an extrusion method,inflation method, and so forth. It is permissible that a certain extentof drawing or stretching be done in this step of forming the nonorientedsheet. The thickness of the sheet at this stage is ordinarily in a rangeof from 0.1 to 3 mm.

The non-oriented sheet is then drawn or stretched along two orthogonalaxes. Although it is preferable that the sheet be first drawn orstretched along its longitudinal axis and then drawn or stretched alonga lateral axis, it may also be drawn or stretched in the reversedsequence or in a simultaneous step along the longitudinal and lateralaxes. A drawing or stretching factor in either of the directions of morethan twice the length is desirable. More precisely, when the drawing orstretching factor is expressed in terms of the areamultiplicationfactor, a value falling within a range of from 9 to 50 times ispreferable. Thus, with a suitable value selected for the drawing orstretching factor depending on the thickness of the non-oriented sheet,the thickness of the resultant product can be adjusted in a desiredrange.

As is well known, all of the drawable or stretchable synthetic resisnshave respective optimum drawing or stretching temperatures. The optimumdrawing or stretching temperature is herein defined asthe temperature atwhich the synthetic resin is drawn at maximum speed with minimum tensilestrength, and yet the molecular orientation in the sheet is neverthelessmaintained. For a resin of crystalline nature, the optimum drawingtemperature is high when the melting point of the resin is high and islow when the melting point of the resin is low. Furthermore, it is foundthat a resin of non-crystalline nature has an optimum drawing orstretching temperature above the glass-transition temperature of theresin.

It should be noted that the optimum drawing or stretching temperature ofa synthetic resin is variable depending on the conditions under whichthe synthetic resin is drawn. Accordingly, the optimum drawing orstretching temperature of the first synthetic resin is also varied incases where only the first synthetic resin exists without being mixedwith others and where the second synthetic resin is mixed with the firstsynthetic resin even if other conditions remain the same. Generally,when the coexisting other synthetic resin is of a higher melting pointthan the synthetic resin in question, the optimum drawing or stretchingtemperature of the synthetic resin will be shifted or extended to thehigher temperature side, and extent of the shifting or extension dependson the mixed amount of the coexisting synthetic resin.

When the non-oriented sheet is drawn or stretched at the optimum drawingor stretching temperature of a component synthetic resin having a lowermelting point or a lower glass transition temperature with the abovedescribed drawing factor, the resultant film-like structure containsdispersed reticulated fibers consisting of the synthetic resin of highermelting point and also with minute voids founded by the inorganic powdercontained in the non-oriented sheet and is reduced in thickness to avalue falling within the range of from 20 to 300 microns;

The accompanying FIGS. 1 and 3 are microscopic photographs of the outersurface of a paper-like film thus obtained, and FIGS. 2 and 4 aremicroscopic photographs of a cross-sectional area of the same film. Allof these photographs clearly show the reticulated structure of thepaper-like film.

Although a synthetic paper consisting of a single sheet of thepaper-like film of the above described structure is the most typical ofthe synthetic papers according to the present invention, it will be alsoapparent that various modifications as described hereinbelow can becarried out within the scope of the present invention.

1. Multiplication of the component synthetic resins:

Both of the matrix phase and the reticulated structural phase may bemade of several kinds of synthetic resins. The nature of theadditionally employed synthetic resins should be substantially the sameas that of the basically employed synthetic resins. However, when bothof the phases employ additional synthetic resins other than the basicsynthetic resins, the difference between the melting points of theadditional resins included respectively in the two phases is notnecessarily more than 15C.

-2. Modification on the filler:

For the purpose of creating the minute voids, rendering or improving thereticulated and fibrillated structure, or improving the writability orprintability of the resultant synthetic paper, the addition of a minuteinorganic powder as a filter is desirable. The above described purposescan be fulfilled to the maximum extent by selecting the kind, grainsize, and the mixed quantity of the inorganic powder or by additionallyusing one or more kinds of inorganic powder. In same specific cases,however, the mixed quantity of the inorganic powder may be reduced to afar smaller value than that above stated (for instance to 1 percent byweight).

On the other hand, in accordance with necessity, the mixed sheet to bedrawn or stretched into the film-like structure may also contain otherfillers of the kinds ordinarily used in this kind of synthetic resincompounds such as a stabilizer, plasticizer, coloring material, organicfiller (in powder form or in fibrous form), foaming agent, and the like.These fillers may be added at the stage of mixing of the componentmaterials into the mixture to be formed into the non-oriented sheet, orthese may be mixed beforehand into the component synthetic resins whichare thereafter mixed to form the above described mixture. I

3. Surface treatment:

Although the paper-like film having the above described structurealready possesses sufficient characteristics to be employed as thepreviously described paper-like layer, the film may be further subjectedto a surface treatment if necessary. Typical examples of the surfacetreatment are a corona discharge treatment, oxidizing flame treatment,and sandblast treatment. A further example of the surface treatmentcomprises treating the surface of the paper-like film with a syntheticresin solution. More specifically, the surface of the film may betreated with a solution which consists of an acrylate-ester polymer (ofa lower alkylester) dissolved in a positive solvent with respect topreferable the synthetic resins forming the paper-like layer (forforming the matrix and/or reticulated structure). The positive solventis herein defined as a solvent having an action at least to swell thesynthetic resins forming the paper-like layer at a treating temperature(of at least 50C) (Reference: Japanese Patent Application No. 89932/68.

4. Lamination:

Although the above described paper-like film in itself can be employedas the paper-like layer, in other words, as a single layer syntheticpaper, it is also possible to laminate a plurality of the paper-likefilm into a multilayer synthetic paper. One example of this, laminationis in the provision of a synthetic paper wherein the above describedpaper-like film or films are bonded on one or two surfaces of a liningfilm. A wide variety of modifications can be considered for this kind ofstructure depending on the existence or nonexistence of drawing orstretching of the lined film, existence or non-existence of fillers, andon the bonding methods of the paper-like film or films. For instance,the synthetic resin structure forming the paper-like layer and anothersynthetic resin structure forming the lining film may be extrudedsimultaneously, or one of the synthetic resin structure may be laminatedby being extruded on the other synthetic resin structure already formedinto a film, so that the non-oriented sheet is thereby formed, and thenon-oriented sheet may be thereafter drawn or stretched into a laminatedsynthetic paper.

The abovementioned lining film may be either one of the ordinarysynthetic film and the paper-like film in the scope of the presentinvention, or if it is required, the film may be a non-synthetic resinfilm such as a cellulose paper or even a metal. The lamination may alsobe carried out without employing any bonding agent as in the case of theextrusion lamination, or when it is required, it may carried out byemploying a bonding layer or through the interposition of an anchorlayer.

Another kind of example of the laminated synthetic paper can be obtainedby providing a low melting point synthetic resin layer on the surfaceofa paper-like layer for rendering a better heat-sealing nature. For thelow melting point synthetic resin layer, the same material as thoseemployed for the aforementioned low melting point synthetic resin orother material having a still lower melting point can be employed.Although various kinds and compositions inclusive of the existence ornonextistence of fillers and with or without the drawing may beconsidered for the low-melting point synthetic resin, it is preferableto provide the low-melting point synthetic resin layer on the substratepaper-like layer before the layer is drawn or stretched, as in the caseof the above described lining film, and then the entire combination ofthe overlaid low-melting point synthetic resin and the substrate can besubjected to a drawing or stretching process.

A typical process for producing the above described synthetic paper isillustrated in FIG. 5. In this production process, a mixture consistingof a high-melting point synthetic resin and a low-melting pointsynthetic resin inclusive of the minute inorganic powder is heated in anextrudable temperature and kneaded therein and is then extruded throughthe slit of a die 3 of the extruder 2. The extruded mixture is thencooled to a required temperature by means of a cooling device 4 and 5 sothat a non-oriented sheet 1 is thereby obtained. in this case, thecooling device may be of any type for roll cooling or water-bath coolingor a combination thereof.

The non-oriented sheet 1 is thereafter heated by means of metal rolls 6and 7 to a temperature required for the drawing or stretching(employment of an infrared ray heating apparatus instead of or incooperation with the above described metal rolls 6 and 7 is alsopossible) and then drawn or stretched in its longitudinal direction totwice its original length by means of two rolls 7 and 8 rotated atdifferent speeds. The sheet thus drawn or stretched in one direction isthereafter fed through rolls 9, 10, and 11 to a tentering machine 12 setto a required temperature, and the width of the sheet is drawn orstretched laterally to more than twice the original width. The laterallydrawn or stretched sheet is then cooled while being held in its drawn orstretched state. The side and longitudinal edges of the sheet aretrimmed, after which the sheet is fed through rolls 13, 14, and 15 andtaken up on a winding roll 16.

The synthetic paper in accordance with the present invention can be usedin substantially all fields of paper appliances except those utilizingthe effect caused by entangled cellulose fibers. For instance, the papercan be used in the fields of wrapping, writing, printing, covering, andtableting, for exhibition purposes such as posters, and for wiping as inthe case of the tissue paper. Furthermore, the paper can be furtherprocessed into a tracing paper, chits, map paper, labels, art coatpaper, photograph printing paper, and other special papers.

EXAMPLE 1 l. Resin mixture:

Polyethylene (Ml 0.95, SG 0.96) parts lsotactic polypropylene (Ml 1.0)18 parts Polystyrene [1;] (toluene 25C) 1.3) 8 parts Diatom earth(average grain diameter 4 6 parts Clay (average grain diameter 1 u) 6parts Titanium oxide (average grain diameter 0.3 p.) 2

parts Melting point of the above described polyethylene is 138C, themelgint point of the isotactic polypropylens is 165C, and the glasstransition point of the polystyrene is 98C. The optimum temperatureranges for drawing or stretching these materials at the drawing orstretching speed of 180 cm/min. are from 133 to 137C, from 150 to 160C,and 120 to 135C, respectively.

2. Treatment The above described mixture was heated, kneaded, andextruded through the die of an extruder at 230C; cooled to a temperaturelower than 40C, and formed into a non-oriented sheet of 0.9-mmthickness.

This sheet was drawn or stretched at 135C in its longitudinal directionso that 4 times the length was obtained and then drawn or stretchedlaterally so that 6 times the width was obtained. The drawing orstretching speed was considerably faster than 180 cm/min. The film thusobtained was cooled in the drawn or stretched condition, and after itslateral edges were trimmed, was wound on a roll.

The film thus produced was found to have following properties inaddition to a high void factor, excellent opacity, whiteness, andsmoothness of the surfaces, and excellent adaptability for writing andprinting. The film had an excellent papery feel and stiffness and couldbe employed in substantially all fields of appliance of conventionalpapers.

Thickness (microns) 38 Apparent density (g/cm)* 0.57

Voids 46.3

Tensile strength (kg/cm) longitudinally 514 laterally 799 Clarkestiffness 4.2

Whiteness 89.9

Opacity 93.3

Surface smoothness (sec.)* 10,000 or more fsinc'iiivaiu'riitifiifirofi'ichitibfi considerable lowering of the density was causedby the existence of the voids.

2 Measured by an Oken-type smoothness tester.

EXAMPLE 1. Resin mixture:

Polyethylene (M1 0.95, 86 0.96) 65 parts l sotactic polypropylene (M11.0) parts Diatom earth (average grain diameter 4 parts (The resinsthemselves are similar to those employed in Example 1) 2. Treatment Theabove described mixture was heated, kneaded, and extruded through thedie of an extruder of 230C, cooled to a temperature lower than 40C, andformed into a non-oriented sheet of 0.7-mm thickness.

This sheet was drawn or stretched at 135C and a drawing or stretchingspeed of 180 cm/min in its longitudinal and lateral directionssimultaneously so that it is extended to 4 X 4 times. The thus obtainedfilm was cooled at the extended condition and the side edge portionswere cut.

Thus produced film was found to have following natures beside of a highvoid factor, excellent opaqueness, whiteness, and smoothness of thesurfaces, and found to be adapted for writing and printing. The film hadalso an excellent papery feeling and a stiffness and could be employedin substantially all kinds of applications adapted for the conventionalpapers.

Thickness (microns) 105 Apparent desnity (g/cm") 0.67

Voids 38.0

Tensile strength (kg/cm) longitudinally 725 laterally 471 Whiteness 89Opacity 97 Surface smoothness (sec.) 570 EXAMPLE 3 l. Resin mixturePolyethylene (M1 0.95, SG 0.96) 56 parts Polystyrene ([1 (toluene 25C)1.3) 16 parts Ethylene/vinyl acetate copolymer (content of vinyl acetate10%, M1= 4) 8 parts Diatom earth (average grain diameter 4p.) 20 partsThe above described polyethylene and polystyrene are the same as thoseemployed in Example 1, and the ethylene/vinyl acetate copolymer has amelting point of 99C and an optimum drawing temperature ranging from toC.

2. Treatment The above described mixture was heated, kneaded, andextruded through the die of an extruder at 230C, cooledto a temperaturelower than 40C, and formed into a non-oriented sheet of 0.9-mmthickness.

This sheet was drawn or stretched at C and a drawing or stretching speedof 180 cm/min first in its longitudinal direction to 4 times itsoriginal length and then in its lateral direction to 7.5 times, and thefilm thus drawn was cooled in the drawn or stretched condition, the sideedges thereafter being trimmed.

The resulting film had the following properties in addition to a highvoid factor, excellent opacity and whiteness, good writability, goodpapery feel, considerable stiffness, and excellent printability and wasfound to be effectively usable in substantially all fields ofapplications of conventional papers.

Thickness (microns) 90 Apparent density (g/cm 0.64

Voids 41.9

Tensile strength (kg/cm longitudinally 580 laterally 613 Whiteness 87Surface smoothness 120 The kinds of the resins are the same as thoseemployed in Example 1. i l 2. Treatment Films obtained after thetreatment set forth in Example 3 (except for drawing rations of 4.4) hadthe follow ing excellent properties.

Parts:

first second third sample sample sample Thickness (microns) 56 5 8 61Apparent density (g/cm") 0.89 0.87 0.80 Voids 11.8 17.1 27.9 Opacity 355 8 86 Surface smoothness 1050 102 98 EXAMPLE 1. Resin mixturePolyethylene (M1= 0.95, SG 0.96) 57 parts Isotactic polypropylene (Ml1.0) 20 parts Polystyrene ([n] 1.3) 8 parts Clay (average grain diameter1y.) parts The kinds of resins employed herein are the same as thoseemployed in Example 1.

2. Treatment A non-oriented sheet was produced through the sam procedureas described in Example 1 and was drawn through an extruder at thefollowing drawing temperatures to 5 times in its longitudinal directionand 7 times in its lateral direction. The sheet thus drawn into afilmlike configuration was then cooled in the drawn condition, and theside edges thereof were trimmed. As a result, it was found that thefilms except that drawn at a temperature higher than 165C had excellentproperties as follows.

Void

Drawing temp. Apparent density factor Opacit 140 0.445 59.5 99.0 1450.494 55.0 98.2 150 0.655 40.5 93.2 155 0.740 32.7 88.0 160 0.895 18.676.5 165 1.03 6.4 58.3 170 (Could not be drawn because the samplemelted) A microscopic photograph of a surface of a synthetic paper drawnat 145C is shown in FIG. 1, and a similar photograph of across-sectional surface of the same synthetic paper is shown in FIG. 2.Furthermore, another microscopic photograph of a surface of a syntheticpaper drawn or stretched at 160C is shown in FIG. 3 and thecross-sectional view is shown in FIG. 4. These photographs were taken bymeans of a scanningtype electron microscope manufactured by NIPPONDENSI-II CO. (JSM 2), with a magnification of 2,400 in all cases.

As is apparent from these photographs, fibrillated fibers are runningthrough the matrix of a film having voids, and inorganic fillers aredispersed within the same matrix.

EXAMPLE 6 1. Resin mixture Polyethylene (M1 0.95, SG 0.96) 70 partsPolybutene-l (M1 1.0, SG 0.92) parts Diatom earth (average graindiameter 4) 10 parts The polyethylene is the same as that employed inExample l, and polybutene-l has a melting point of l 11C and an optimumdrawing temperature ranging from 85 to 100C.

2. Treatment The above described mixture was heated, kneaded, andextruded through the die of an extruder so that a non-oriented sheet of0.8-mm thickness was obtained. This sheet was thereafter drawn orstretched at 130C to 4 times the original length in the longitudinaldirection and then to 4 times the original width in the lateraldirection and was cooled in the drawn or stretched condition, the sideedges thereof being thereafter trimmed. The film thus obtained had thefollowing properties and was found to be usable in the application fieldof conventional paper.

Thickness (microns) 45 Apparent density (g/cm) 0.89 Voids- 12% Opacity65% EXAMPLE 7 1. Resin mixture Polyethylene (M1 0.95, SG 0.96) partsEthylene/vinly acetate copolymer (SG 0.93) content of vinyl acetate is10 wt. 20 parts Diatom earth (average grain diameter of 4p.) 10

parts The above resins are the same as those employed in Example 3.

2. Treatment I An opaque film was produced with the above materials inaccordance with Example 6 and was found to be suitable for the samepurposes and applications as those for conventional papers.

Thickness (microns) 49 Apparent density (g/cm) 0.82

Voids 18.8%

Opacity 67% I claim:

1. A synthetic paper comprising a layer in the form of a substantiallyopaque film, said layer comprising av film matrix of a first syntheticresin stretched along at least one axis thereof, a second syntheticresin split into fibrillated fibers of reticulated structure dispersedin said film matrix of said first synthetic resin, and said layer havinga fine inorganic filler in an amount less than 60 wt. of said first andsecond synthetic resins, whereby a void percentage of more than 10percent is attained in said layer.

2. A synthetic paper as defined in claim 1 wherein only said layer isemployed for obtaining said synthetic paper.

3. A synthetic paper as defined in claim 1 wherein a .plurality oflayers, at least one of which is the firstmentioned layer, are employedfor obtaining said synthetic paper.

4. A synthetic paper as defined in claim 1 wherein said first and secondsynthetic resins are polyethylene and isotatic polypropylene,respectively.

5. A synthetic paper as defined in claim 1 wherein said first and secondsynthetic resins are polyethylene polystyrene and isotacticpolypropylene, respectively.

6. A synthetic paper as defined in claim 1 wherein said first and secondsynthetic resins are polystyrene ethylene/vinyl acetate copolymer andpolyethylene, respectively.

7. A synthetic paper as defined in claim 1 wherein said first and secondsynthetic resins are polystyrene and polythylene, respectively.

8. A synthetic paper as defined in claim 1 wherein said first and secondsynthetic resins are polybutene-l and polyethylene, respectively.

9. A synthetic paper as defined in claim 1 wherein said first and secondsynthetic resins are ethylene/vinyl acetate copolymer and polyethylene,respectively.

2. A synthetic paper as defined in claim 1 wherein only said layer isemployed for obtaining said synthetic paper.
 3. A synthetic paper asdefined in claim 1 wherein a plurality of layers, at least one of whichis the first-mentioned layer, are employed for obtaining said syntheticpaper.
 4. A synthetic paper as defined in claim 1 wherein said first andsecond synthetic resins are polyethylene and isotatic polypropylene,respectively.
 5. A synthetic paper as defined in claim 1 wherein saidfirst and second synthetic resins are polyethylene + polystyrene andisotactic polypropylene, respectively.
 6. A synthetic paper as definedin claim 1 wherein said first and second synthetic resins arepolystyrene + ethylene/vinyl acetate copolymer and polyethylene,respectively.
 7. A synthetic paper as defined in claim 1 wherein saidfirst and second synthetic resins are polystyrene and polythylene,respectively.
 8. A synthetic paper as defined in claim 1 wherein saidfirst and second synthetic resins are polybutene-1 and polyethylene,respectively.
 9. A synthetic paper as defined in claim 1 wherein saidfirst and second synthetic resins are ethylene/vinyl acetate copolymerand polyethylene, respectively.